1. Field of the Invention
The present invention relates to an R-T-B based sintered magnet and a rare-earth alloy as its material.
2. Description of the Related Art
An R-T-B based sintered magnet, one of the most prominent high-performance permanent magnets (which is sometimes called a “neodymium-iron-boron-based sintered magnet”), has excellent magnetic properties, and is used in motors, actuators, and various other applications.
An R-T-B based sintered magnet is comprised of a main phase consisting essentially of a compound with an R2Fe14B type crystal structure (i.e., R2Fe14B compound phase), an R-rich phase and a B-rich phase. Basic compositions of R-T-B based sintered magnets are disclosed, for example, in U.S. Pat. Nos. 4,770,723 and 4,792,368, the entire disclosures of which are hereby incorporated by reference. An R-T-B based sintered magnet has a higher maximum energy product than any of various other magnets, but is expected to have its performance (in remanence, among other things) further improved. For instance, even just 1% increase in remanence should have an immense industrial value.
To increase the remanence of a sintered magnet, the density of the sintered magnet (which will be sometimes referred to herein as a “sintered density”) needs to be as close to its true density as possible. However, if the sintering process is carried out either at a higher temperature or for a longer time to increase the density of an R-T-B based sintered magnet, then the sintered density will increase but the crystal grains thereof will have excessively big sizes to cause a decrease in coercivity, which is a problem. Particularly if an “abnormal grain growth” occurred to produce giant crystal grains (main phases) locally, then the square ratio Hk/HcJ of the demagnetization curve would decrease so much as to cause various inconveniences when such a magnet is actually used.
That is to say, in the prior art, it is difficult to increase the sintered density of an R-T-B based sintered magnet without sacrificing its coercivity. Also, even if sintering conditions that would contribute to striking an adequate balance in resultant performance could be found, their margins should be too narrow to manufacture R-T-B based sintered magnets with excellent performance constantly on an industrial basis.
Japanese Patent Application Laid-Open Publications No. 61-295355 and No. 2002-75717 disclose techniques of suppressing the abnormal grain growth by nucleating a boride on a gain boundary with the addition of Ti, Zr or any other element that produces the boride. According to the methods disclosed in Japanese Patent Application Laid-Open Publications No. 61-295355 and No. 2002-75717, the sintered density can be increased with the excessive increase in crystal grain size avoided (i.e., with the decrease in coercivity minimized).
According to the methods disclosed in Japanese Patent Application Laid-Open Publications No. 61-295355 and No. 2002-75717, however, a boride phase with no magnetic force (i.e., B-rich phase) is present in the sintered magnet, and the main phase that produces the magnetism (i.e., R2T14B type compound phase) has a decreased volume percentage, thus resulting in decreased remanence.